1. Technical Field
This invention relates to industrial process control instrumentation, more particularly, adaptive filtering techniques.
2. Background Art
It has been known for many years that vortices are developed in a fluid flowing past a non-streamlined obstruction. It also has been known that with certain arrangements vortices are developed by alternate shedding at regular intervals from opposite edges of the obstruction to form corresponding rows of vortices. Such vortices establish a so-called von Karman "vortex street," which is a stable vortex formation consisting of two nearly-parallel rows of evenly-spaced vortices traveling with the flow stream.
In a von Karman vortex street, the vortices of one row are staggered relative to those of the other row by approximately one-half the distance between consecutive vortices in the same row. The spacing between successive vortices in each row is very nearly constant over a range of flow rates, so that the frequency of vortex formation is correspondingly proportional to the velocity of the fluid. Thus, by sensing the frequency of vortex formation it is possible to measure the fluid flow rate. Devices for that purpose are often referred to as vortex meters or vortex flowmeters.
Various types of vortex meters have been available commercially for a number of years. Typically, these vortex meters comprise a vortex-shedding body mounted in a flow tube together with a sensor for detecting the generation of vortex formation. Sensors used to detect the vortices often include diaphragms which fluctuate in response to alternating differential pressure variations generated by the vortices. The pressure applied to the diaphragms is transferred to a sensor or transducer which then produces electronic signals responsive to differential pressure variations applied to the diaphragms. This differential pressure measurement is used, in turn, to measure the frequency of vortex formation and ultimately the fluid flow rate or velocity.
The sensor produces an analog sinusoidal voltage signal with frequencies ranging from 0 Hz to 3200 Hz. Various types of electronic components are used to condition and process the vortex sensor signal and thereby measure the flow rate. In many applications, the flowmeter circuitry is constrained by cost and, in addition, power consumption in order to adhere to industrial instrumentation standards.
One such type of signal conditioning component is an electrical filtering circuit for sifting out noise signals associated with the acoustic, electrical, and mechanical vibration sources existing in the ambient flowmeter surroundings. Vortex sensor generated signals distorted by these noise signals result in errors in counting the vortex shedding frequency and, consequently, in measuring the flow rate. To alleviate this error, the signal passes through a bandpass filter which passes a specified band of frequencies while attenuating all signals outside the band. Due to the variable frequency range of the vortex signal arising from different size meters and process conditions, the filter needs to be tailored for a particular application.
To avoid having to redesign the filter for each different frequency range, the prior art teaches of various adaptive filtering techniques for adapting or tuning the filter automatically to follow the vortex signal. Tracking filters and filter groups are commonly used techniques.
Tracking filters have a frequency pass band that tracks or follows the changing frequency of a signal applied to its input. These filters consist of an active filter and a feedback means to control a preselected frequency pass band of the filter in accordance with the frequency of the output signal from the filter. However, tracking filters can lock on to noise signals rather than the vortex signal, thereby giving a false measure of the flowrate.
Filters groups consist of a plurality of electronic filters having a control mechanism which switches on the appropriate filters in response to the measured vortex shedding frequency. This technique is costly, requiring complex circuitry and consuming a large amount of power.
Both of these techniques are of limited value for industrial applications requiring low power consumption, low cost component construction, and reliable performance. Accordingly, there exists a need for an improved apparatus for adaptively filtering the vortex sensor signal used in industrial process instrumentation.
It is an object of this invention to provide a reliable technique for filtering a variable frequency analog vortex sensor signal.
It is a further object of the invention to provide a filtering technique as described above constructed with low cost components and utilizing low power consumption.
It is a further object to provide a filter as described above which is dynamically tunable to the frequency of the input signal while avoiding locking onto noise signals.
It is another object to provide a filter as described above which provides a clock-controlled tunable means for altering the corner frequencies of a band-pass filter in response to a variable-frequency analog sinusoidal input signal.
Other general and specific objects of this invention will be apparent and evident from the accompanying drawings and the following description.